39 research outputs found
In situ spectroelectrochemistry and colour measurement of a complementary electrochromic device based on surface-confined Prussian blue and aqueous solution-phase methyl viologen
This article was published in the journal, Solar Energy Materials and Solar Cells [© Elsevier B.V.] and the definitive version is available at: http://dx.doi.org/10.1016/j.solmat.2011.11.052The fabrication, in situ spectroelectrochemistry and colour measurement of hybrid electrochromic
devices (ECDs) based on a surface-confined metal hexacyanometallate – Prussian blue (PB,
containing the iron(III) hexacyanoferrate(II) chromophore) – and aqueous solution-phase methyl
viologen (N,N´-dimethyl-4,4´-bipyridylium) are described. In the ECDs, the initial (‘off’) bleached
state is set with PB in its reduced form and the methyl viologen as the di-cation. Switching to the
coloured state (‘on’), forms the mixed-valence iron(III) hexacyanoferrate(II) chromophore on
oxidation of iron(II) hexacyanoferrate(II), with simultaneous reduction of the methyl viologen dication
to form a mixture of the radical cation monomer/dimer. Using the Commission
Internationale de l'Eclairage (CIE) system of colorimetry, the colour stimulus of such ECDs and the
changes that take place on reversibly switching between the colourless and coloured states have
been calculated from in situ visible region spectra recorded under electrochemical control. The
concentration of the solution-phase methyl viologen and its diffusion to the cathode controlled both
the proportion of surface-confined (reduced) PB that is switched to the blue form and the overall
ECD changes. For the ECDs’ ‘on’ states, the CIELAB 1976 color space coordinates for a D55
illuminant were L* = 60, a* = 3 and b* = −46, and L* = 49, a* = 9 and b* = −59, respectively for 5
and 10 mM methyl viologen solution concentrations. The low a* and high (negative) b* chromaticity coordinates quantified the overall ECD colour stimulus of the ‘on’ state as being deep
blue, with a broad absorption across the visible spectral region. Combination of the methyl
viologen system in the ECDs served to remove the green tint perceived in single film PB. CIELAB
1976 colour space coordinates showed that the ECDs were fully transparent and nearly colourless in
the ‘off’ states, with L* = 100, a* = 1 and b* = 1. The changes in the transparency were 83.0% (5
mM methyl viologen) and 93.1% (10 mM methyl viologen) between the ‘off’ (bleached) and ‘on’
(coloured) states of the ECDs